Environmental Engineering Reference
In-Depth Information
TABLE 7.1 Summary of Redox Properties of Adatom-Modified Pt(111)
Electrodes in 0.5 M H
2
SO
4
Solution (Except Where Otherwise Stated). All Potentials
are Given vs. RHE and Measured at 50 mV/s
S [Batina et al., 1989]
E
p
. 0.8 V in 10 mM NaF at
pH 9-10
a
S(0) O S(VI)
þ
6e
2
u
max
¼ 0.33
a
Ge [G ´mez et al., 1992]
E
an
¼ 0.73 V
b
Ge(0) O Ge(IV)
þ
4e
2
As [Feliu et al., 1988]
E
p
¼ 0.56 V
c
q
P
0
¼ q
Pt
þ
q
As
As(0) O As(III)
þ
3e
2
u¼ q
As
/(3
241)
u
max
¼ 0.33
Se [Feliu et al., 1993a]
E
an
¼ 1.08 and 0.98 V
d
E
cat
¼ 0.79 V
q
P
0
¼ q
Pt
þ
4
q
Se
Se(0) O Se(IV)
þ
4e
2
u¼ q
Se
/(4
241)
u
max
¼ 0.33
Sn [Xiao et al., 2002]
Broad peaks in 0.05 M H
2
SO
e
Sn(0) O Sn(IV)
þ
4e
2
u¼ q
Sn
/(4
241)
u
max
¼ 0.32
Sb [Climent et al., 1998;
Feliu et al., 1988]
E
p
¼ 0.485 V
f
q
P
0
¼ q
Pt
þ
2
q
Sb
Sb(0) O Sb(II)
þ
2e
2
u¼ q
Sb
/(2
241)
u
max
¼ 0.33
Te [Feliu et al., 1993b]
E
p
¼ 0.83 V
g
q
P
0
¼ q
Pt
þ
q
Te
Te(0) O Te(IV)
þ
4e
2
u¼ q
Te
/(4
241)
u
max
¼ 0.25
Pb [Feliu et al., 1991]
E
p
¼ several peaks between
0.5 and 0.6 V
h
q
P
0
¼ q
Pt
þ
2
q
Pb
Pb(0) O Pb(II)
þ
2e
2
u¼ q
Pb
/(2
241)
u
max
¼ 0.33
Bi [Feliu et al., 1991]
E
p
¼ 0.62 V
i
q
P
0
¼ q
Pt
þ
2
q
Bi
Bi(0) O Bi(II)
þ
2e
2
u¼ q
Bi
/(2
241)
u
max
¼ 0.33
a
The S oxidation process involves desorption of S adatoms, and the formation of H
2
SO
4
species has been tentatively pro-
posed. The maximum coverage was determined from Auger electron spectroscopy (AES) and low energy electron diffrac-
tion (LEED) measurements. When the irreversible adsorption of sulfur is performed in an oxygen-free Na
2
S solution, the
formation of more compact S adlayers on Pt(111) has been identified from AES and LEED measurements [Sung et al.,
1997, 1998].
b
The Ge adatoms do not remain adsorbed after oxidation, and the characteristic voltammetric profile of the blank is recov-
ered after about 2 - 3 cycles up to 1.2 V. The Ge reduction process does not take place in a well-defined peak.
c
At near-saturation coverages, the As redox process splits, with E
p2
¼ 0.54 V. Partial desorption of As can be achieved by
increasing the positive potential limit to values higher than 0.85 V.
d
The second Se oxidation peak at 0.98 V appears at u
Se
. 0.17. At near-saturation coverages, this peak becomes broader
and shifts to positive potentials, leading to an apparent increase in current density of the peak at 1.08 V. In order to desorb
the Se adatoms, the upper potential limit has to be increased to 1.3 V, where a new, small oxidation peak is observed, tenta-
tively assigned to the formation of Se(VI) species.
e
At saturation coverage, Sn oxidation starts at 0.54 V and shows a broad wave, centered at 0.86 V, up to 1.3 V. The reduction
process is centered at 0.56 V. At lower Sn coverages, a new peak appears at 0.61 V, which becomes sharper and is displaced
to 0.56 V as the coverage is further decreased. This second process has been tentatively assigned to Sn
!
Sn(I).
f
At high coverages (u
Sb
.
0.2), the formation of a Pt - Sb alloy has been proposed, concomitant with a broadening of the
Sb process at 0.49 V and the appearance of a new peak around 0.43 V.
g
Only at saturation coverage is there a certain degree of irreversibility, which is reflected in DE
p
¼ E
an
2 E
cat
¼ 45 mV. A
new oxidation process is observed at 1.01 V that leads to progressive dissolution of the adsorbed adlayer. This process has
been tentatively ascribed to the formation of soluble Te(VI) species. The corresponding reduction process is observed
around 0.9 V.
h
A continuous decrease in the amount of adsorbed Pb is observed in successive voltammetric cycles up to 0.6 V. For this
reason, the stoichiometry of the Pb process has been inferred from measurements in 0.1 M NaOH solutions.
i
The dissolution of the adsorbed Bi only takes place at potentials around 1 V.
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